This invention relates to improvements in synthetic plastic material moulding processes and apparatus and relates particularly but not exclusively to improvements in processes and apparatus for moulding foamable synthetic plastics materials such as polyurethane foam plastics materials or elastomeric materials.
In presently known processes for manufacturing moulded plastics material articles, feedstock materials required to produce the synthetic plastics material product are stored in separate storage vessels and are delivered at high pressure to a mixing and delivery head via separate supply lines. The feedstocks are intimately mixed due to the impingement of the feedstocks at high pressure in the mixing head immediately prior to delivery and pouring of the mixed feedstocks into a mould. Where the pouring of the mixed feedstock to a succession of moulds is intermittent, the mixing head provides for the recirculation of individual feedstocks to the respective storage vessels while a subsequent mould is being positioned in relation to the mixing head prior to the pouring of the mixed feedstock into the mould. The recirculation of the feedstocks is designed to maintain, as near as possible, a homogenous mix of the individual feedstocks and to maintain as near as possible a uniform temperature of the individual feedstocks at any point in time, in the feedstocks storage and delivery system.
The feedstocks are delivered to the mixing head by means of variable displacement hydraulic pumps driven at constant speed by constant speed (i.e., at constant speed under constant load) electric motors. The output of the pumps may be varied by adjusting the displacement of the pump by manual, mechanical, hydraulic or electrical means. Such adjustment of the feedstock pumps may be required to compensate for inherent variations in the volumetric output of the pumps caused by mechanical tolerances in the pumps or caused, in the long term, by wear of the pump or, in the shorter term, by variations in the speed of operation of the pump caused by variations in the speed of operation of the electric motor driving the pump. The variation in the speed of operation of the "constant speed" electric motors is caused by variations in the load placed on the motors due to variations in the physical properties (such as specific gravity, viscosity etc.) of the feedstocks being pumped. The physical property variations may be caused by variations in temperature or composition, for example, of one or more of the feedstocks. In the case of the production of foamed plastics material products, particularly foamed polyurethane materials, small variations in the output of the pumps can produce relatively large variations in the physical characteristics of the resultant foamed polyurethane product.
In the moulding of foam polyurethane products from polyol and isocyanate feedstocks, for example, it is possible for the volumetric output of the pumps to vary by .+-.1.5% for the polyol feedstock and by .+-.3.0% for the isocyanate feedstock. In the absence of any corrective measures, constant predetermined volumes of feedstock are not delivered via the mixing head to the mould. This results in the production of moulded products, over a period of time, with a variation in characteristics or properties which may be outside defined product tolerances. A high incidence of reject products can therefore result.
Such adjustment may also be required if different feedstock mixes are required in successive moulds of a series of different moulds or if different feedstock mixes are required in different sections of an individual mould to produce for example a foamed plastics material article having sections with different hardness or resiliency characteristics such as in a motor vehicle seat or back rest cushion.
The adjustment of the output of the pump by adjusting the displacement of the pump cannot be readily accomplished to compensate for inherent variations in output of the pump as the mechanical adjustment of the pump does not provide for sufficiently accurate volumetric adjustment. The volumetric output of the pumps may be manually varied to compensate for such change but such variation will still result in the production of unacceptable products as the variations are made only when noticeable changes in the physical characteristics of the product occur. The testing of the physical characteristics of a product is time consuming. Furthermore, as the products, in the case of foamed products, continue to cure after removal from a mould until a full cure is achieved, any testing of the characteristics must be made at a specified time after removal of a product from a mould for any meaningful comparison of products to be made and to determine whether any variation of pump output is required.
A high incidence of reject or unsatisfactory products can therefore result when the flow rates of the constituent feedstock materials delivered to the mould vary as a result of changes in process operating parameters and these flow rates are not adjusted instantaneously so as to remain within the find tolerances required to consistently produce a moulded foam end product with the desired material properties.
U.S. Pat. No. 4,571,319 (Baluch et al.) discloses a method and apparatus for manufacturing polymer articles which have different properties in different regions of the articles. However, this system has a number of disadvantages in production processes where the material properties of the moulded foam end product are particularly sensitive to the volumes of the various constituent feedstock materials combined. In such production processes, it is necessary to accurately and continuously control the flows of the various feedstock materials so as to consistently produce a moulded end product possessing the desired material properties.
The Baluch system disclosed in U.S. Pat. No. 4,571,319 is a hydraulically powered system in which the feedstock pumps supplying the various constituents are driven by hydraulic motors. In this system, the speed of operation of the hydraulic motors (and thus the speed of operation and output of the feedstock pumps) is varied by means of servo-valves. However, a servo controlled hydraulically powered moulding system of the type disclosed by Baluch lacks the degree of accuracy and responsiveness to fine and rapid adjustments often required in order to provide desired degree of control over the flow rates of the various constituent materials to consistently produce an end product with the desired properties.
Furthermore, the control system in Baluch et al. relies upon an analog tachometer and a digital encoder which are mechanically coupled to each motor/pump combination to provide feedback signals to the controller indicating pump speed and the amount of pump rotation. This system is based upon the premise that the tachometer signal as well as the encoder pulse rate are accurate representations of the material flow rate. Hence, in the Baluch system, the actual material flow rate is approximated by the tachometer and encoder signals. In particular, the signals from the tachometer and encoder represent the pump speed which is assumed to represent material flow rate. Therefore, in the Baluch system, the control system measures the theoretical flow rate by assuming that a given pump speed will accurately produce a certain flow rate of material. However, in practice, variations in individual feedstock temperatures, specific gravity and viscosity have the effect that the feedstock material flow rate will not be constant and/or repeatable for a given pump speed. Moreover, were in a feedstock pump will alter its output characteristics and result in a change in the pump output for a given speed over time. Thus, it is evident that this system cannot make compensations for such variations in individual feedstock temperatures, specific gravity and viscosity or for ware in individual feedstock pumps over time.